Using parameters in SAM file in DefinitionUri

Hi, FYI in my SAM file this works:
StageName: !Ref "Env"
DefinitionUri: "swagger-dev.yml"

But this doesn't work:
StageName: !Ref "Env"
DefinitionUri: !Sub "swagger-${Env}.yml"

And this also doesn't work:
StageName: !Ref "Env"
DefinitionUri:
Fn::Join:
- ''
- - "swagger-"
- !Ref: "Env"
- ".yml"

The error I get is:
"FAILED - Transform AWS::Serverless-2016-10-31 failed with: Invalid Serverless Application Specification document. Number of errors found: 1. Resource with id [Api] is invalid. 'DefinitionUri' requires Bucket and Key properties to be specified"

My parameter is defined as:

Parameters:
Env:
Description: The environment id.
Type: String
ConstraintDescription: Can contain only one of the supported environment ids.

[sanathkr]

You should use aws cloudformation package command if you specify local files. Unfortunately package doesn't parse intrinsic functions. So it doesn't concatenate the !Sub before uploading to S3. You should upload your files to S3 yourself and follow https://github.com/awslabs/serverless-application-model/blob/master/HOWTO.md#using-intrinsic-functions to specify Swagger URI using !Sub

[sanathkr]

I don't see aws cloudformation package command supporting full CloudFormation Intrinsics spec anytime. It is a good use-case to use intrinsics for local paths, but very hard for a local tool to provide full fidelity support to intrinsic functions